Errors in the Kinetics of Disproportionation of Aqueous Superoxide

Many publications have mechanisms that include rate constants for the disproportionation of aqueous superoxide that are far from correct. The well-established rate law is rate = k HO2•,HO2• [HO 2 • ] 2 + k HO2•,O2•– [HO 2 • ][O 2 •– ] with k HO2•,HO2• = 8.3 × 10 5 M –1 s –1 and k HO2•,O2•– = 9.4 × 10 7 M –1 s –1 at 25 °C. The relative concentrations of HO 2 • and O 2 •– are determined by the p K a of HO 2 •, which is 4.8. At high pH, the rate of disproportionation becomes so slow that its measurement is unreliable because of catalysis by impurity metal ions. Thus, only an upper limit of 0.3 M –1 s –1 can be assigned to the rate constant for disproportionation via two O 2 •– radicals. However, there are many publications that use a value of 8.3 × 10 9 M –1 s –1 for k HO2•,HO2• . There are also several publications that use k HO2•,HO2• = 1.7 × 10 7 M –1 s –1 . Kinetic simulations performed with these incorrect k HO2•,HO2• values can produce significantly incorrect predictions of the time dependence of [HO 2 • ]. Production of singlet O 2 is also erroneously invoked in several publications.